A coherent amplify-and-forward relaying demonstrator without global phase reference

In this work we present a demonstrator for coherent linear amplify-and-forward (AF) relaying. Two source/destination pairs communicate with the help of three half-duplex relays in a two-hop manner. We implemented a coherent gain allocation scheme that suppresses inter-user interference at each destination and does not require a global phase reference at the relays. In order to assess the performance of the scheme, we calculated the signal-to-interference ratio (SIR) at both destination nodes. Inter-user interference arises when the local oscillator phases of the relays change independently during one transmission cycle, e.g. due to phase noise. Consequently, the shorter the time between reception and retransmission at the relays, the more accurate the interference suppression. Measurement results show that our demonstrator is able to suppress inter-user interference at both destinations reasonably well.